1. Field of the Invention
The present invention relates to an optical fiber assembly having a hermetic seal portion, which is used in a parallel optical transmission module and to a method for making the same.
2. Description of the Related Art
Typically, an optical module includes in its interior an optical element such as a waveguide channel, a light-emitting device, and is used for optically coupling in its interior the optical element to an optical fiber introduced from the outside. In a process of optically coupling the optical element included in the optical module to the optical fiber, a feedthrough portion for introducing the optical fiber into the optical module is formed through a wall of a package of the optical module. Since the package of the optical module must be hermetically sealed as a whole, hermetic sealing of the feedthrough portion is an important factor which influences the reliability of the optical module.
Further, a multi-core array of optical fibers is used for connecting the optical fibers to a planar waveguide channel and for coupling the optical fibers to an array of light-emitting devices, and is one of the elemental components essential for a parallel optical transmission module used in a high-density transmission system. In a process of optically coupling the multi-core array of optical fibers, i.e. an optical fiber assembly to the optical element of the parallel optical transmission module, the feedthrough portion for introducing the optical fiber assembly into the parallel optical transmission module is formed in the wall of the package of the parallel optical transmission module. Since the package of the parallel optical transmission module must be hermetically sealed as a whole, hermetic seal of the feedthrough is also an important factor which influences the reliability of the parallel optical transmission module.
In conventional hermetic sealing technology for an optical fiber assembly, there has been such a method that a part of the optical fiber assembly is metallized (coated with metal) after the coated film of that part having been peeled off, and then the metallized portion is directly soldered onto a package to seal hermetically. However, the method is of a little practical use, since the optical fiber assembly is liable to be cut off, for example, when external force is applied to the optical fiber assembly positioned at the boundary of a seal portion outside the package.
In order to solve the problem, the following method has been conventionally adopted. As shown in FIG. 1 and FIG. 2, after forming an optical fiber assembly with a hermetic seal portion 401 by soldering a metallized portion 412 of the optical fiber assembly 410 into a metal pipe 420 to seal the two hermetically, both the metal pipe 420 of the optical fiber assembly with the hermetic seal portion 401 and a package of a parallel optical transmission module not shown in the figures are hermetically sealed together. In this method, as the solder for sealing the optical fiber assembly 410 and the metal pipe 420 used is a high melting point solder which is not influenced by the temperature to be employed for sealing the package.
However, the following problem will occur even if this method may be employed. In order to construct the optical fiber assembly with the hermetic seal portion 401 as shown in FIG. 1 and FIG. 2, it is necessary to insert the metallized portion 412 of the optical fiber assembly 410 into a flat feedthrough hole 421 formed in the central part of a cylindrical metal pipe 420 and to enter solder from an edge of the metal pipe 420 into the feedthrough hole 421.
According to such structural restraint, it is practically necessary to inject solder into the feedthrough hole 421 while the metal pipe 420 is heated in such a state that the metallized portion 412 of the optical fiber assembly 410 is exposed out of the edge of the metal pipe 420, and then to draw back the optical fiber assembly 410 or the metal pipe 420 in order to receive the metallized portion 412 in the interior of the feedthrough hole 421 while the metal pipe 420 is kept heated so as not to allow the injected solder to solidify. In this method, the solder injected into the feedthrough hole 421 is liable to get localized since the optical fiber assembly 410 requires stirring in the metal pipe 420 during the soldering process. Consequently, there has been often such a case where a stable hermetic seal cannot be obtained.
In order to solve the problem, the following two methods may be taken into consideration. The first method is that the metallized portion 412 is positioned near the edge of the metal pipe 420 and is soldered at the location in order to solder the optical fiber assembly 410 without stirring it. However, since the optical fiber assembly 410 positioned outside the metal pipe 420 apart from the soldered site is not protected with the metal pipe 420 in this method, its bending strength is remarkably weak.
The second method is such that a paste-like solder is placed by injection into the feedthrough hole 421 of the metal pipe 420; the optical fiber assembly 410 is positioned in the prescribed place; and the metal pipe 420 is then heated. However, since flux components may normally be mixed in the paste-like solder in this method, the mixed flux must be removed using a flux removal agent after the soldering process. Therefore, the coating of the optical fiber assembly 410 is liable to be damaged by the flux removal agent.